The present disclosure pertains to the field of internal combustion engines, including engines for motor vehicles, railways, ships, aircraft, or electrical power generation.
This disclosure pertains to internal combustion engines that operate far more efficiently than conventional engines. The principles set forth herein can be used in both spark-ignition (SI) engines typically operating on gasoline (petrol), ethanol or natural gas, or on compression-ignition engines, which typically are diesel engines.
The engine literature describes a number of factors that affect engine efficiency. These can be divided into theoretical limits based on the second law of thermodynamics, namely the temperature differential (gradient) that determines efficiency in the Carnot cycle, and compression ratio which is the most pertinent variable in Otto cycle efficiency. Other factors are important, including mechanical factors such as friction and chemical factors such as fuel properties. Fuel properties depend on the chemical makeup of the fuel, the stoichiometry, vaporization of liquid fuels, and other factors, including the combustion temperature, ignition energy and ignition delay, flame propagation velocity, and completeness of combustion.
Internal combustion engines are heat engines, whose behavior can be described in the ideal limit by the laws of thermodynamics. The work and thermal energy of any heat driven process can be described by the first law of thermodynamics as:Qin=Wout+Qout 
where Qin is thermal energy put into the engine, and mechanical energy, or work is Wout. A cyclic heat engine, even in the ideal limit, cannot completely convert the net heat input into work output, so some of the input heat energy has to be dissipated into the environment as waste heat Qout. The thermal efficiency of a cyclic heat engine is defined as:
            η      th        ≡                  W        out                    Q        in              =      1    -                  Q        out                    Q        in            
where ηth is a dimensionless efficiency factor. This is a performance measure of a device that uses thermal energy, such as an internal combustion engine.
The theoretical maximum efficiency of any heat engine is given by the Carnot theorem, which posits that the theoretical maximum efficiency of any heat engine depends on the difference between hot and cold temperature reservoirs in an ideal thermodynamically reversible engine. This maximum efficiency in a Carnot engine is defined to be:
      η    th    ≤      1    -                  T        C                    T        H            
where TC is the absolute temperature of the cold reservoir, and TH is the absolute temperature of the hot reservoir. Therefore, efficiency in a Carnot engine is a factor of the temperature gradient between the hot and cold reservoirs.
The Otto cycle is another ideal thermodynamic cycle that relates engine efficiency of internal combustion spark-ignition engines to compression ratio. The geometry of Otto cycle employs two adiabatic and two constant volume processes. Otto cycle efficiency, which assumes perfect gas law behavior, can be expressed as:
      η    th    =      1    -          1              r                  y          -          1                    
where r is the volume compression ratio, and γ=Cp/Cv, the specific heat ratio, of heat capacity at constant pressure (CP) to heat capacity at constant volume (CV). A similar formula for diesel engines relates compression ratio (and combustion expansion ratio) to efficiency in diesel (compression ignition) engines. The specific heat ratio is also known as the “isentropic expansion factor.” The specific heat ratio of the air-fuel mixture γ varies with temperature and the heat capacity of the fuel vapor, but is generally close to the air value of 1.4. When using this standard value, the cycle is called an “air-standard cycle.” Because γ is always greater than 1, engine efficiency in the Otto cycle is directly related to compression ratio. Therefore, high compression ratio engines will operate more efficiently than a lower compression ratio engine, all other factors being equal.
Temperature control in engines is also an important factor affecting engine efficiency. The Carnot cycle suggests that the higher the temperature after the ignition at top dead center (TDC) of the piston in the cylinder (i.e., the highest temperature in the engine), the larger the temperature differential will be, which leads to greater efficiency. However, real world inefficiencies include the lack of complete mixing of the fuel with air, the rate of combustion, and the air/fuel ratio required for effective ignition. Most engines operate at a near stoichiometric air/fuel ratio. Combustion under these conditions creates excess heat that is not converted into mechanical work. This excess heat must be rejected using a radiator or through the exhaust. The high combustion temperatures created also create undesirable NOx emissions.
Temperature control in modern engines is usually accomplished by a cooling jacket surrounding the engine, transporting heat to a heat exchanger (radiator) that rejects excess heat to the environment and maintains the engine within operating temperature limits. The use of a conventional radiator in such a fashion is termed herein as external cooling. Most modern internal combustion engines are liquid (or water) cooled (externally cooled) using either water or some other liquid coolant, which circulates through the engine and runs through the heat exchanger. Alternatively, some engines are characterized as “air cooled,” typically because they lack a radiator. Instead, most air cooled engines have additional fins integral with the engine block or cylinders to convect and radiate heat away from the engine.
Even in the most efficient liquid or air cooled conventional engines, the requirement to shed heat through the cooling system significantly decreases engine efficiency. Approximately 40% of engine heat is dissipated in the radiator or cooling fins, which is lost energy, some part of which could still theoretically be useable as mechanical energy. Thus, reducing this heat loss, and converting excess heat to useful mechanical energy, is an important unmet need in engine design. Conventional automobiles are only about 20% efficient at converting the energy in gasoline to mechanical energy. The remaining 80% or so of the energy in the fuel is lost to the environment through the cooling system and heat exchanger (radiator) and as exhaust heat. Thus, if the heat loss through the radiator (or otherwise dissipated to the environment) could be substantially reduced, engine efficiency could be substantially improved.
The compression ratio in engines which use fuels like gasoline or natural gas is limited by the need to control engine knock, which is caused by pre(auto)-ignition of the fuel prior to the desired ignition from firing of the spark plug. During pre-ignition, fuel ignites during the compression stroke in an uncontrolled fashion due to the high temperatures generated in the cylinder during compression. Such pre-ignition wastes energy and could lead to engine damage if uncontrolled. To avoid engine knock, conventional spark ignition engines are generally limited to an effective compression ratio of about 10:1, with up to 12:1 possible with more expensive high octane fuel.
An additional factor affecting engine performance is the air (oxygen) to fuel ratio. Stoichiometric air provides one mole of molecular oxygen per mole of carbon and 0.5 mole of molecular oxygen per mole of hydrogen in the fuel. The amount of air for true stoichiometric oxygen is dependent on the exact chemical makeup of the fuel, but is approximately 14.7:1 weight/weight (w/w) for gasoline and diesel engines (i.e., 1 gram of fuel to 14.7 grams of air). Engines are typically run rich during cold start and high load operation, but when run rich, there will be non-combusted fuel and thus wasted energy and additional air pollution. Engines normally run most efficiently at about a stoichiometric mixture, but there are theoretical bases for efficient engine operation under lean conditions, of greater than stoichiometric oxygen.